1. Field of the Invention
The present invention relates to a method for controlling a character (e.g., a morphology and a yield) of monocotyledonous plants, and a transformed plant produced by controlling a character thereof.
2. Description of the Related Art
A technique of producing plants, particularly major cereals, having a desired character (e.g., a desired morphology and an increased yield) by controlling a character (e.g., a morphology) of the plant, is considerably useful in the agriculture field.
For example, if rice is modified into a short culm (semidwarf) morphology, abnormal elongation of stems or leaves of the plant due to supply of a fertilizer is suppressed. In this case, resistance to lodging due to physical force, such as wind or the like, is improved, resulting in resistance to a high level of fertilizer. Further, the proportion of fertilizers or anabolites distributed to stems and leaves is relatively decreased in association with the suppression of elongation, while the proportion of fertilizers or anabolites distributed to the growth of spikes is relatively increased. As a result, the growth of spikes is promoted in the modified plant, thereby increasing the yield. Alternatively, a variant of rice, which is modified to have erected leaves, has an improved level of light interception as compared to wild type rice. For such a variant rice, an area under an individual plant required for receiving a necessary amount of sun light can be reduced. As a result, the amount of cultivation per unit area can be increased.
However, when conventional methods are used to produce plants having such a morphology as erected leaf, short culm, and the like, the growth of spikes of the plant is inhibited, resulting in a reduction in yield.
To date, no method for producing a useful variety having a desired character, such as a beneficial morphology (erected leaf, short culm, or the like), no reduction in yields, and the like, has been known. Therefore, there is a demand for a method for producing useful plant varieties having a desired character, and plants, seeds, plant cells, and whole plants produced by the method.
As a plant hormone relating to control of the morphology of plants, brassinosteroid is well known. Brassinosteroid is a general term for plant growth regulatory hormones having a steroid lactone structure, including brassinolide.
In 1979, brassinolide was isolated as a novel plant growth-promoting factor from the pollen of rape (Brassica napus) and identified as a novel type of steroid lactone. It was thereafter found that brassinolide-like steroid compounds (referred to as brassinosteroid) occur at very low concentrations in all plant species examined (for review, see Mandava, Ann. Rev. Plant Physiol. Plant Mol. Biol. 39 (1988), 23-52). Initial studies of the physiological action of brassinolide showed that this particular factor (i) accelerated the germination and growth of plant seedlings at low temperatures, (ii) promoted the increase of cell size and elongation by induction of a longitudinal arrangement of cortical microtubule and cellulose microfilaments on the surface of cells, (iii) promoted xylem differentiation by amplifying the tracheal elements, (iv) resulted in significant increase in the dry weight of plants and their fruits, (v) promoted leaf unrolling and enlargement, (vi) induced H+ export and membrane hyperpolarization characteristic for auxin induced cell growth, (vii) inhibited the division of crown-gall tumor cells and radial growth of stems, (viii) repressed anthocyanin production in light-grown plants, (ix) inhibited the de-etiolation induced, e.g. by cytokinin in the dark, (x) promoted tissue senescence in the dark, but prolonged the life-span of plants in the light, and (xi) induced plant pathogen resistance responses to numerous bacterial and fungal species (Mandava, Ann. Rev. Plant Physiol. Plant Mol. Biol. 39 (1988), 23-52).
Following the initial isolation of and physiological studies with brassinolides, numerous brassinosteroid compounds, representing putative biosynthetic intermediates, were identified in different plant species. Because the in vivo concentration of these compounds was found to be extremely low, efforts had been made to develop methods for chemical synthesis of these compounds (for review, see: Adam and Marquardt, Phytochem. 25 (1986), 1787-1799).
In order to be able to demonstrate that brassinosteroids can indeed be used as potential growth regulators of plants and to exploit the possible advantages and potentials of these substances, genes involved in synthesis and signal transduction of brassinosteroid have been isolated and analyzed.
For example, a d61 mutant lacking OsBRI1, which is a brassinosteroid receptor-like gene, was isolated from rice. Useful characters, such as erected leaf, semidwarf, and the like, were observed for the d61 mutant or plants in which the function of OSBRI1 is suppressed by an antisense nucleic acid, though the number of grains per spike was reduced and the size of grains was reduced, and the like, i.e., adverse effects on yield was confirmed (FIG. 1).
It might be possible that mutation relating to synthesis and/or signal transduction of brassinosteroid can be compensated for by exogenous brassinosteroid (e.g., by spraying or applying brassinosteroid) to avoid the adverse effects of the mutation. However, since d61 has a mutation in OsBRI1 (a brassinosteroid receptor-like gene), it is not possible to compensate for the phenotype thereof with exogenous brassinosteroid.
As a modified plant responsive to exogenous brassinosteroid, for example, plants having a mutation in a gene for a brassinosteroid synthesis system may be illustrated. However, plants having a mutation in a gene for a brassinosteroid synthesis system have been believed to not be able to be used to produce a good character according to previous findings described below.
For example, the cpd gene encoding a cytochrome P450 protein involved in brassinosteroid synthesis in plants has been identified for Arabidopsis (WO97/35986). It was reported that when grown in soil under white light, the size of cpd mutant plants, which have mutation in the cpd gene, was 20 to 30-fold smaller than that of the same age wild type plants. It was also reported that exposure to light induced greening and chloroplast differentiation in the periderm of mutant roots and resulted in a further inhibition of cell elongation, leading to an overall reduction of the length of petioles, leaves, inflorescence-stems and flower organs.
Choe et al. reported that the size of Arabidopsis plants having a mutation in DWF4, which is a cytochrome P450 monooxygenase enzyme involved in brassinosteroid biosynthesis, was several-fold smaller than that of wild type plants, i.e., superdwarfism, and that the plants are infertile (Sunghwa Choe et al., The Plant Cell, vol. 10, 231-243, February, 1998).
Choe et al. (The Plant Journal, vol. 26, 573-582, June, 2001) reported that the ratio of seed yield to plant height was not improved in the transgenic Arabidopsis overexpressing the DWF4 gene. Therefore, according to the above-described results for Arabidopsis, it has not been believed that modification and/or overexpression of DWF4 can be used in order to produce modified plants having a useful character.
Hong et al. (Hong Zhi et al., Proceedings of the 2002 Annual Meeting of the Japanese Society of Plant Physiologists, p. 224) and Mori et al. (Masaki Mori et al., Proceedings of the 2002 Annual Meeting of the Japanese Society of Plant Physiologists, p. 225) reported that substantially no leaf sheath was formed in rice plants having a mutation in OsDWARF which catalyzes oxidation activity at position C-6 of brassinosteroid, i.e., superdwarfism was shown along with extreme deformity. Therefore, similar to Arabidopsis, it has not been believed that modification and/or overexpression of a gene for a brassinosteroid biosynthesis or signal transduction system can be used in order to produce modified rice plants having a useful character.
Accordingly, although brassinosteroid was known as a plant hormone for controlling the morphology of plants, no method for using a gene for a brassinosteroid synthesis system was developed to produce plants having a useful character.
The present invention provides a method of producing a plant having a useful character using modification and/or overexpression of a gene for a brassinosteroid synthesis system, contrary to previous findings. The present invention also provides a plant, a plant seed, a plant cell and plant tissue produced by the method.
An object of the present invention is to provide a method of producing a useful plant variety having a desired morphology, and a plant, a seed, a plant cell, and plant tissue produced by the method, and the progeny of the plant. In the current situation there is no method of producing a plant having a desired morphology (particularly, short culm and/or verticality leading to improved light interception) and having substantially no adverse effect (e.g., reduction of yields) due to modification by controlling a character (e.g., a morphology) of plants.
According to previous findings, it was not believed that modification of a gene for a brassinosteroid synthesis system and/or signal transduction system allows for production of a useful plant variety having a useful character, such as a desired morphology. On the contrary, it is known that if a mutation is introduced into a gene for a brassinosteroid synthesis system and/or signal transduction system, a resultant mutant plant has a superdwarf morphology or an adversely affected yield. Such an adverse effect on yields has not been believed to be able to be removed. It was not believed that overexpression of a gene for a brassinosteroid synthesis system and/or signal transduction system allows for production of a plant having a useful character, such as an increase in yield.
Thus, it was not believed that modification and/or overexpression of a gene for a brassinosteroid synthesis system and/or signal transduction system allows for production of a plant having a useful character.